1. Field of the Invention
The present invention relates to automobiles, particularly to those of a rear wheel drive type (hereinafter referred to as a RWD system) which employed a floor fabricated to be low and lengthwise flat on which to mount a baggage room, a deck body, etc. for receiving cargo.
2. Description of the Prior Art
Generally with trucks and similar motor vehicles for cargo transport and delivery service, it is desired that the floor on which cargo is loaded be flat, wide and as low as practical in reference to the ground level.
A low floor renders driver ease of cargo loading and unloading and therefore serves to improve not only the workability of trucks or the like but also the efficiency of cargo loading and unloading.
A flat floor with no structural protrusion in the area between the wheel rooms (arch-profiled portions for rear wheels) allows smooth cargo arrangements thereon and effective utilization of the floor space in the luggage room. Further, securing a wide floor space within the range in which the floor undergoes dimensional restriction due to the length of a car body, particularly the width thereof, makes it feasible to effect one cycle of loading with many cargoes, and improve the efficiency of transport.
Reflecting the reality that the ratio of aged motor drivers has been rising in recent years with the number of female drivers likewise increasing, it may fairly be said that low-floor structural engineering will meet mounting demands to better cargo loading and unloading onto and from a deck body, particularly for such drivers in charge of via-truck or commercial vehicle delivery service.
With motor vehicle (a) such as a small truck, for example, having a high deck body as shown in FIG. 1, floor (b) comes in a flat structure. The motor driver in charge of such a motor vehicle is required to hold each cargo (c) up to the deck floor level during loading. Loading with said motor vehicle entails intensive fatigue to the driver. Where no assistant is available, the driver must effect loading in two steps; in the first step, some cargoes (c) are placed on the deck floor, and the second step then comes with his jumping up thereon for cargo arrangement. Such cargo loading is really hard work.
The motor vehicle with its deck floor lowered to approximately the waist height of a driver by decreasing the radius of rear wheel (f) as shown in FIG. 2 allows ease of loading, compared with the high deck body type referred to above. However, with said motor vehicle, the driver is not allowed to ascend floor (b) with the cargo in his arm. He is required to separately effect loading and arrangement on floor (b) with no improvement of efficiency.
Lowering the deck floor to the level shown in FIG. 3 at which a driver can ascend the deck in one step will not only remarkably relieve the driver of fatigue but also will improve the workability of a motor vehicle and the efficiency of loading. With a lowered deck body, the driver is allowed to effect cargo loading and subsequent cargo arrangement on floor (b) simultaneously, following his raising of the cargo up to a reduced deck floor level. Thus, with a low-deck body type motor vehicle, one man can easily undertake the job of loading.
To date, the motor vehicles used for cargo transport service are usually provided with a front engine, RWD system and a rear engine. The RWD system keeps driving rear axle shafts under the luggage room or the deck body at the rear part of a vehicle.
Designed to propel a vehicle by driving the rear wheels, the system is suited for use with those motor employed for heavy cargo transport. Taking into account severe conditions of heavy cargo transport and the cost of automobile production, the RWD system for commercial vehicles is generally of a rigid type (axle suspension type) featuring robust axles and inexpensiveness.
As shown in FIG. 4, the RWD type of the rigid type comprises rear axle case (R) including widthwise centered enlarged round structural member (d) which houses the differential gears (hereinafter referred to as the differential) and tubular members (e) (hollow members), each extending monolithically from the widthwise centered round structural member (d) in the widthwise direction, rear axle shafts (not shown in the figure)--rear driving axle shafts--, each being inserted into and running through the interior of tubular member (e) of rear axle case (R) with one end engaging with the differenrtial side gear and the other end coupled to rear wheel (f), and so forth, both rear axles moving up and down, together with the rear axle case (R), following vertical movements of both rear wheels (f). Further, each suspension mechanism (h) (a rigid axle type suspension system) comprising a leaf spring, joined to side member (g)--a component making up the frame--, and other attachments are secured to each tubular member (e) holding said rigid rear axle therein, whereby the loads respectively of body (i), cargo, etc. which are applied to the leaf spring, are suspended. Side member (g) is a rigid structural member making up the frame, each frame comprising a pair of said side members extending lengthwise from the front to the rear of a vehicle and which are kept apart with a wide space maintained widthwise between the two. Particularly, in the vicinity of each rear axle, each of said side members (g) is located above end portions (j) of tubular member (e) of rear axle case (R). Floor (k) is laid over a pair of side members (g). The motor vehicles, each using the RWD system with the conventional rigid type rear axles possessed the various restrictions enumerated below, resulting in the increase in above-ground height (H) of floor (k), particularly in the level of rear axle case (R).
(1) With the RWD system, rear axle case (R) of a monolithic construction moves up and down, following vertical movements of rear wheels (f). Therefore, to prevent a collision between floor (k) on side of the body (1) upheld with suspension mechanism (h) and widthwise centered round portion (d) so molded at the center of rear axle case (R), clearance (C1) is required.
(2) Relative vertical movements of left and right rear wheels (f) cause rear axle case (R) to move up and down with one rear wheel (f) acting as a fulcrum upon its coming back into contact with the ground earlier than the other rear wheel. With this taken into account, it is necessary that side member (g) located above each of both widthwise end portions (j) of tubular member (e) have a height to allow rear axle case (R) sufficient to move up and down within a given amplitude but to prevent collision with tubular member (e).
(3) To limit the oscillation amplitude of tubular member (e) referred to in (2) above, buffer rubber (1) is provided underneath of side member (g), facing the tubular member (e). The buffer rubber (1) must be mounted with specified clearance (C2) provided over the surface of the tubular member (e) so that the buffer member (1) will not only allow the tubular member (e) to move up and down within a given amplitude but also to restrict vertical movements thereof within the amplitude. In addition, the buffer rubber (1) maintains considerable height (H1), and to enable the side member (g) to accept the impact force from a collision with said tubular member (e), the buffer rubber (1) is mounted directly to the side member (g), thereby necessitating increasing the clearance (C3) between the top surface of the tubular member (e) and the bottom of the side member (g).
(4) To assure widthwise centered round portion (d) of rear axle case (R) is maintained free from collision with some obstacle on the ground, it is necessary to maintain clearance (C4) between the underneath of the round portion (d) and the ground surface.
(5) Wheel radius (R1) affects the above-ground hight of the rear driving axle shafts and other attachments of the RWD system. So far, various proposals have been raised with these above-remarked factors taken into account, to lessen the deck floor height.
Unlike the conventional side members which used to be formed with a cetain height kept uniform above ground, to lessen the floor level of a cab and other parts of a vehicle, a pair of side members (g) shown in FIGS. 5 and 6 curve upward in the neighborhood of the rear axle case (R) as if to cross over it with specified clearances and heights secured for those portions which incur the various restrictive factors. However, such a proposal as quoted above came forth, specifing the above-ground height of rear axle case (R) vicinity which is approximately the same as compared with the conventional type frames, and thus failing to lower the level of floor (k) located over rear axle case (R) to that of other parts. Following this proposal, it is unfeasible to effectively lessen the floor level of a motor vehicle.
By decreasing the apparent height of buffer rubber (1) with the buffer rubber (1) secured, in a parallel relationship with a bracket outside side member (g) biased inward slightly in the width-wise direction and which is shown in FIG. 4, to the underneath of the bracket, it becomes possible to lower the level of floor (k) to some extent. However, this means does not make much difference, with floor (k) held over side member (g) while a specified clearance is maintained beyond a certain range within which said side member (g) keeps clear of the floor (k). The extent which this means allows lowering floor (k), is not more than the height of the buffer rubber (1). With this taken into considered, such means is not regarded to be substantially effective.
Meanwhile, another means has in the past been attempted to lower the above-ground height of the deck body (b) by shifting down the range, through which the side member keeps clear of a deck floor, with the radius of rear wheel (f) reduced as shown in FIG. 2. This other means gives rise to the problem that there occurs some loss of tractive force, and reduction in brake capacity. To compensate for this problem, it is conceivable that radius-reduced rear wheels could be employed into service, using a double wheel design. Such compensation creates a problem in that wheel change is made difficult within the limited wheel room, especially with the double-wheel system having two wheels kept in use while locked to a hub. Another problem arises with this compensation in that a motor vehicle must carry two spare wheels of two different radii, therefore, the vehicle must provide for additional spare wheel space, resulting in an increase in vehicle weight or causing some inconvenience.
It is also conceivable that the level of the floor is lowered with the rear axle case and other attachments which come with the RWD system, by introducing a front wheel drive system (hereinafter referred to as the FWD system). As compared with the RWD system, the FWD system is inferior in its capacity to produce tractive force. The attempt to make up for insufficient tractive force with greater load applied to the front wheels will increase imbalance between the front and rear wheels the load distribution when the vehicle is duly loaded, unlike the case where the vehicle is not loaded. Remarkable reduction in brake performance will result, requiring an auxiliary brake system for the security of safety. In view of the above, the FWD system proves to be unfit for commercial vehicles.